The present invention relates generally to integrated circuits (ICs), and more particularly, to impedance-matching circuits.
An integrated circuit (IC) includes multiple source circuits that drive corresponding sets of load circuits, transferring power to the load circuits. To improve efficiency of the operation of the IC, maximum power should be transferred from a source circuit to the corresponding load circuits. It is generally understood that maximum power is transferred from the source circuit to the corresponding load circuits when the output impedance of the source circuit matches the input impedance of the load circuits. A mismatch between the output and input impedances results in a reduction of the power transferred to the load circuits, which in turn leads to a leakage of power to other components of the IC.
A source circuit such as a radio frequency power amplifier (RFPA), which drives at least one load circuit such as an antenna, operates at a very high frequency. At high frequencies, a mismatch between an output impedance of the RFPA and an input impedance of the load circuit(s) results in a greater reduction in the power transferred from the RFPA to the load circuit(s) as compared to the reduction in the transferred power from a source circuit operating at a low frequency. This reduction in the transferred power leads to a greater leakage of the power to the RFPA and other components of the IC, which can damage the RFPA as well as the other components. Hence, an impedance-matching circuit is needed to accurately match the output impedance of the RFPA with the input impedance of the load circuit(s).
The impedance-matching circuit is connected between the RFPA and the load circuit(s). The impedance-matching circuit includes at least one inductor-capacitor (LC) filter that includes an inductor and a capacitor connected in series with each other. The LC filter is connected in parallel with the RFPA. The output impedance of the RFPA is based on an impedance of the LC filter. The impedance of the LC filter is determined in such a way that the output impedance matches the input impedance of the load circuit(s).
Typically, the RFPA includes multiple transistors, at least one of which operates at a fundamental frequency and outputs an RFPA output signal that is amplified at the fundamental frequency. During amplification of the RFPA output signal, the RFPA introduces a nonlinear distortion in the RFPA output signal. Further, asymmetric fabrication of the transistors may introduce corresponding delays in generation of the RFPA output signal. The nonlinear distortion and delays lead to an introduction of spurious harmonic frequencies (i.e., integral multiples of the fundamental frequency) in the RFPA output signal. Typically, the output impedance of the RFPA is based on the fundamental frequency. However, due to the introduction of these spurious harmonic frequencies, the output impedance of the RFPA is affected by the harmonic frequencies, causing the output impedance to fluctuate, leading to a mismatch between the output impedance and the input impedance of the load circuit(s).
It is desirable that a voltage level of the RFPA output signal is equal to a voltage level corresponding to the fundamental frequency. However, due to a constructive interference in signals of the RFPA output signal at the harmonics frequencies, the voltage level of the RFPA output signal equals a sum of voltage levels corresponding to the harmonic frequencies and the fundamental frequency, which results in an increase in the voltage level of the RFPA output signal. Thus, the RFPA may not accurately output the RFPA output signal at the desired voltage level. Furthermore, the increase in the voltage level of the RFPA output signal stresses the RFPA transistors, which can damage the transistors (i.e., decreases the ruggedness of the RFPA).
One way to prevent these problems is to include at least one resistor-capacitor (RC) filter in the impedance-matching circuit. The RC filter is connected in cascade with the RFPA. The output impedance of the RFPA is based on an impedance of the RC filter. The impedance of the RC filter is such that the output impedance matches the input impedance of the load circuit(s). Further, the RC filter filters the RFPA output signal and reduces a voltage level at each of the harmonic frequencies of the RFPA output signal, which reduces the voltage stress on the transistors. However, since the RC filter is connected in cascade with the RFPA, a voltage level of the fundamental frequency also is modified, which is not desirable. Further, the inclusion of multiple RC filters for multiple harmonic frequencies increases the complexity of the impedance-matching circuit.
It would be advantageous to have an impedance-matching circuit that reduces a voltage level corresponding to at least one harmonic frequency of an RFPA output signal and does not change the voltage level of a fundamental frequency of the RFPA output signal.